Estimating soil moisture content by measuring the resistance between electrodes which are placed in the soil is well known. Moisture in the soil provides a conductive path between the electrodes. As the soil becomes more moist, the resistance between the electrodes decreases. In dry soil the resistance between the electrodes is high. A response curve providing soil moisture as a function of resistance for a particular soil type can be determined through testing. Thus, by burying electrodes in the soil and monitoring the resistance between the electrodes, soil moisture content can be estimated. In practice, however, a number of other factors effect the resistance between the electrodes.
In their most basic form, resistive soil moisture sensors consist of two electrodes which are placed in direct contact with the soil. U.S. Pat. No. 3,882,383 entitled SOIL MOISTURE SENSING SYSTEM issued to Matlin is exemplary. Unfortunately, sensors with electrodes in direct contact with the soil are adversely effected by variations in both soil particle size and soil moisture conductivity. Since soil particle size and soil moisture conductivity vary greatly from place to place, such sensors are difficult to work with and often provide relatively inaccurate data.
More advanced soil moisture sensors include a housing filled with a filter medium which shields the electrodes from direct contact with the soil. U.S. Pat. No. 5,179,347 entitled ELECTRICAL SENSOR FOR SENSING MOISTURE IN SOILS issued to Hawkins is exemplary. The filter medium in which the electrodes are embedded contains particles of uniform size. The use of the filter medium alleviates the problem associated with variations in soil particle size. The sensor may also include a gypsum buffer tablet to address the problem associated with variation in soil moisture conductivity. However, there is another factor which effects the accuracy of the sensor. This and other known types of soil moisture sensors are substantially temperature dependent. At any given soil moisture content, as temperature decreases, resistance between the electrodes increases. Consequently, if the soil temperature varies while logging data, the resulting data is inaccurate.
Overcoming the temperature dependence problem would allow development of a number of useful applications of soil moisture sensors. One particular area where soil moisture sensors hold great promise is agriculture. Soil moisture sensors could be used in conjunction with automatic irrigation systems to minimize water waste and runoff. Further, by using such sensors in conjunction with data loggers, agronomists could profile changes in soil moisture over time. Such loggers could be placed in remote sites where frequent, long term measurement would otherwise be impossible.
It is known to make a manual temperature compensation based on surface temperature. Typically, a person must visit the measurement site at regular intervals and adjust the logging or measuring device according to the then current surface temperature. This method has at least two major drawbacks. First, surface temperature is often very different from soil temperature. Consequently, error is introduced into the temperature compensation calculation as a result of the difference between surface temperature and soil temperature at the depth of the sensor. This problem becomes more pronounced as the sensor is placed deeper in the soil. Second, data loggers take measurements at regular intervals, there very purpose being to obviate the need for a person to be at the logging site to take measurements. In order to use surface compensation, a person must visit the site frequently. Such frequent visits substantially raise the cost of running the data logging device and still produce relatively inaccurate data. In practical terms, the need for frequent visits nullifies the advantages of using a data logger.
Of course, there are many other uses for soil moisture sensors apart from their use in conjunction with data loggers and automatic irrigation systems. It is therefore desirable to have a soil moisture sensor which is compensated for changes in temperature.